fix: handle null response output text

[he-yufeng]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

• Allow ResponseOutputText.text to be None when the API returns a nullable output_text.text item.
• Make Response.output_text skip null text items instead of failing or appending a non-string value.
• Make responses.parse(...) leave null output_text items unparsed while still parsing later non-null output text items.
• Add regression coverage for both Response.output_text and the structured parse path.

Additional context & links

Fixes #3063.

I know the repository is generated; this PR is intended to pin down the runtime behavior and regression tests for the nullable API payload shape.

Verification:

• PYTHONPATH=src python -m pytest tests\lib\responses\test_responses.py -q
• python -m ruff check src\openai\types\responses\response_output_text.py src\openai\types\responses\response.py src\openai\lib\_parsing\_responses.py tests\lib\responses\test_responses.py
• python -m pyright src\openai\types\responses\response_output_text.py src\openai\types\responses\response.py src\openai\lib\_parsing\_responses.py tests\lib\responses\test_responses.py
• python -m ruff format --check src\openai\types\responses\response_output_text.py src\openai\types\responses\response.py src\openai\lib\_parsing\_responses.py tests\lib\responses\test_responses.py
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use v1-compatible validation in test helper

In the test-pydantic-v1 nox session, noxfile.py installs pydantic<2 and runs the whole pytest suite, but this helper now calls Response.model_validate(...) unconditionally. The SDK's BaseModel does not shim model_validate for pydantic v1, so that CI path will fail with AttributeError before these regressions run. Use the repo's compatibility helper or Response.parse_obj(...) here instead.

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